The Greek situation highlights the difficulty of maintaining a financial union among essentially 28 autonomous governments. The EEC is missing a fully functional political union and other key components needed to facilitate full monetary integration, e.g., a Euro Bond.
Would an exit from the Euro necessarily mean an exit from the EEC? Not necessarily it seems. Though not painless by any means, Greece could go to back to a Drachma floating relative to the Euro, but otherwise remain in the EU economic community. That would result in Greek citizens getting a politically more accessible haircut by the back door via inflation and permit some of the debt to be monetized. This would be no worse for the central EU powers than an outright default. It would simply result in different oxen being gored. At the same time it would keep Greece, for the time being at least, in the Western European orbit. The critical problem being faced now is how to maintain bank liquidity and sufficient Euro reserves to accommodate trade. That's a problem that won't go away if Greece was to return to the drachma, but it might not get much worse.
Whenever politicians are faced with seemingly intractable difficulties, the solution that offers up the least political pain has to be considered a likely candidate for implementation. If Merkel can get Schaeuble to bend over one more time, then Greece will continue to suckle at the EU teat. Otherwise I view a return to the drachma as a foregone conclusion.